The present disclosure relates to processes of preparing crystalline polyester (CPE) microparticles for 3D (3 dimensional) printing. More specifically, the present embodiments provide processes of preparing crystalline polyester (CPE) microparticles with large spherical particles (>20 microns) for selective laser sintering (SLS) 3D printing.
The selective laser sintering (SLS) technique for additive manufacturing (3D printing) uses a rasterized laser to “scan” over a bed of polymer powder, sintering it to form solid shapes in a layer-wise fashion. The material used for SLS is typically powdered polymers, either alone or in composite form. A selection of specifications and capabilities to meet various needs of downstream applications provides the impetus to develop new materials for 3D printing via the SLS process.
Selective Laser Sintering (SLS) 3D printing technology manufactures plastic parts by using a laser as the power source to sinter consecutive layers of polymeric powder. A problem that limits this technology from wide-ranging industrial scope is the narrow variety of applicable polymers. To date, only a few commercial polymers have been successfully applied to this technology mainly comprised of crystalline polyamides (PA), such as PA-11 or PA-12 and some limited use for other materials such as PA-6, thermoplastic polyurethanes (TPU) and polyether block amides (PEBA). Amorphous resins, elastomers or other more flexible materials such as polypropylene (PP) and polyethylene (PE), and higher performance materials crucial to broadening the material properties of 3D parts cannot be used. This limitation is due to the restricted requirement that a material must be crystalline and have a sharp melting point and re-crystallization point of approximately 30° C. to 50° C. difference in temperature.
In a SLS system, a CO2 laser beam is used to selectively fuse or sinter the polymer particles deposited in a thin layer. Local full coalescence of polymer particles in the top powder layer is necessary as well as adhesion with previously sintered particles in the layers below. For crystalline or semi-crystalline polymers usually used in SLS processing, this implies that the crystallization temperature (Tc) should be inhibited during processing for as long as possible, or at least for several sintered layers. Thus, the processing temperature must be precisely controlled in-between melting (Tm) and crystallization (Tc) of the given polymer. This meta-stable thermodynamic region of undercooled polymer melt is called the ‘sintering window’ for a given polymer. The sintering window between onset points of Tc and Tm is about 30° C. to about 40° C. FIG. 1 demonstrates the differential scanning calorimetry (DSC) spectrum for PA-12 SLS powder. (Source: Schmid, et. al., “Polymer Powders for Selective Laser Sintering (SLS)”; ETH-Zürich, 2014.)
Polymer properties that are desired for successful SLS applications include the particles shape and surface of the SLS materials/powders. The more spherically shaped the polymer particles are, the more free-flowing properties they exhibit. Typically, a relatively non-spherical particles could potentially have a negative effect on flow and packing efficiency. This is a desired characteristic for the SLS materials as they are distributed on the part bed of an SLS machine by roller or blade systems and will not be compacted. To date, the currently available commercial SLS powders, such as Nylon PA-12, that are produced by precipitation processes described in Appl. Sci. 2017, 7, 462, are typically non-spherical shaped, so called “potato-shaped” particles. Particles obtained from cryogenic milling are also inadequate for SLS processing, because the cryogenic milled powders flow-ability generates low density and poor part bed surface in SLS machine.
Therefore, there is a need of more rigid or more flexible materials than the currently used polyamide (PA-6, PA-11 and PA-12). Additionally, there is a need for polymeric materials with lower temperature (Tc and Tm), such that lower power requirements is needed for the 3D printer, and processes of making such polymeric materials.